In a multi-leveled gradation display with the use of an image display device employing a two-valued display method, such as a display device with a plasma display panel, a sub-field method has conventionally been used. In the sub-field method, one field is divided into a plurality of sub-fields each of which has a predetermined weight of luminance. Gradation display is obtained by controlling turn-on/off of cells for each sub-field. For example, to achieve 256-level gray scale, one field is divided into 8 sub-fields. In this case, weight of luminance assigned to each sub-field is 1, 2, 4, 8, 16, 32, 64, and 128. When an 8-bit digital signal comes into the device having the above-described sub-fields, each bit of the incoming signal is assigned, in the order of the least significant bit, to the eight sub-fields. The 256 levels are obtained by turning the cells ON in combinations of the eight sub-fields. The luminance provided by the 8 sub-fields are visually accumulated in the eyes, a viewer sees half tones on the display (for example, see Plasma Display Handbook, pp. 165-177, Heiju Uchiike and Shigeo Mikoshiba, Kogyo Chousa Kai Shuppan). The arrangement of the eight sub-fields in one field has a specific limitation; the eight sub-fields may be arranged in the order of increasing the weight of luminance (hereinafter referred to as an ascending coding), as shown in FIG. 1A, or in reverse, they may be arranged in the order of decreasing the weight of luminance (hereinafter, a descending coding), as shown in FIG. 1B.
There have been many suggestions on a method and device capable of reducing noise in an image signal to improve the signal-to-noise (S/N) ratio. Such a method and device is also employed for an image display device having a plasma display panel (for example, see Japanese Patent Unexamined Publication No. 2001-36770); in particular, a frame-cyclic noise reduction method is well known as being highly effective (see Multidimensional Signal Processing for TV Image, p. 190, Takahiko Fukinuki, Nikkan Kogyo Shimbun). Generally, image signals have a strong autocorrelation between frames, whereas noise components contained in the image signals have no autocorrelation. Making use of the characteristics above, a frame-cyclic noise reduction device averages images by frame to reduce noise. However, a motion picture area has a weak autocorrelation between frames; when the averaging process is carried out on the motion picture areas, motion picture images themselves are also averaged. This introduces a fuzzy image or an after-image such as “tailing”, degrading resolution. To address the problem above, a practical frame-cyclic noise reduction device is disclosed in, for example, Japanese Patent Unexamined Publication No. H06-225178. Prior to the averaging, the device detects a motion picture area from an image signal and controls a level of the averaging (hereinafter, a cyclic amount) according to the amount of movement of the detected area. FIG. 2 is a circuit block diagram illustrating a structure of a conventional frame-cyclic noise reduction device. The device detects a motion picture area from a differential signal between frames, and determines the cyclic amount according to an amount of movement; for a motion picture area, cyclic amount k is determined to be small (0≦k≦1) to suppress an after-image, on the other hand, for a still picture area, cyclic amount k is determined to be large to reduce noise.
According to the aforementioned device, however, reducing noise and suppressing a fuzzy contour of motion picture are in a trade-off relation. Therefore, it has been difficult to simultaneously improve both noise reduction and fuzziness suppressing since noise reduction is often traded off for suppressed fuzziness in a motion picture area, or vice versa.
In addition, an image display device employing the sub-field method often causes inconveniences; a viewer often sees a disturbance in gradation display when the eyes follow movement of an image, which is known as dynamic false contour. As another phenomenon, the edge portion of an image becomes blurred (hereinafter the phenomenon is referred to as sub-field fuzziness). For example, Japanese Patent Unexamined Publication No. 2002-229504 addresses the problems above.
Here will be described how the sub-field fuzziness occurs. FIG. 3 shows the turned-on state, with the passage of time, of the sub-fields arranged in the ascending coding of FIG. 1A, and distribution of visual intensity on the retina of human eyes. When pixels A through E represent gray level 63, as shown in FIG. 3, pixels A through E turn on at the sub-fields having weight of luminance of 1, 2, 4, 8, 16, and 32. When the screen shows a still picture, a viewer sees the picture with a line of sight that is fixed, and therefore the luminance generated by the sub-fields having the weights of 1, 2, 4, 8, 16, and 32 is accumulated onto a same position on the retina of the eyes. As a result, the viewer recognizes the image with a uniform visual intensity. On the other hand, when the screen shows a motion picture that is moving toward left, as shown in FIG. 4, the viewer's eyes follow the movement, moving the line of sight to the left. At this time, the luminance given from the sub-fields is diagonally accumulated on the retina, as shown in FIG. 4. This causes variations in visual intensity, providing the motion picture with a fuzzy edge, that is, inviting a poor resolution in the edge portion of motion pictures. This is the sub-field fuzziness mentioned above, which often occurs, regardless of the ascending coding, descending coding, or any other arrangements of the sub-fields, in an image display device employing the sub-field system.
When a viewer watches a motion picture shown on the display device, the motion picture and movement of the line of sight generally have a strong correlation; hereinafter the description will be given on the assumption that the line of sight follows the moving direction of the motion picture on the screen.
The sub-field fuzziness occurs, as described above, at a sharp edge portion in a display area. Actually, when a motion picture, for example, shot by a TV camera is shown on the screen, the edge portion of the motion picture area often becomes blurred. In this case, when the sub-fields are arranged in the ascending coding, as shown in FIG. 5, the sub-field fuzziness observed at an edge portion of the motion picture area on the side where the gradation level is reducing along the moving direction is noticeably getting worse, compared to the blurred edge of the original image. In contrast, when the sub-fields are arranged in the descending coding, as shown in FIG. 6, the sub-field fuzziness observed at an edge portion of the motion picture area on the side where the gradation level is increasing along the moving direction is noticeably getting worse, compared to the blurred edge of the original image. Furthermore, the sub-field fuzziness observed in the motion picture area is more conspicuous, when the edge portion of the original input image is badly blurred, or when the motion picture moves at a higher speed. Such a sub-field fuzziness persists regardless of the ascending coding, descending coding, or any other arrangements of the sub-fields.
In the noise reduction with the use of the aforementioned frame-cyclic noise reduction device, a blurred edge portion of a motion picture area has often amplified the sub-field fuzziness, resulting in deterioration in image quality.
The present invention addresses the problems above. It is therefore the object of the invention to provide an improved method and device of frame-cyclic noise reduction, which prevents the sub-field fuzziness from becoming worse and, at the same time, effectively reduces noise.